I was playing as a Rebel Pilot, returning to Hoth Echo Base in my T-47, with a human in the rear-gunner seat. We had sustained some damage in bringing down an AT-AT and returned to the base to get some quick healing. I flew into the base, and landed safely infront on the repair driod. After the ship was fully repaired, I pressed X to take off again... only instead of flying up, up and away, we surprisingly sank below the level.

The bottom layer of the level (normally snow) looked like clouds in the sky... and you could make out the tunnel structures as we were sinking. As soon as the "Return to the Battle" message played, I kicked in the thrusters, and aimed the sinking T-47 towards the sky (snow), figuring the concrete base structures would kill me instantly. Much to my passanger's surprise, we broke through the snow and ended our subterrianian adventure.... returning safely to the "normal" battle area.

I will try to recreate this in the future, and update here if it happens again.